SgField return type added support for "url_template"

Revolver(Screening Room) adds fields to entities that have a return type of
"url_template".  These fields are strings that point to a url.  Added schema
support for these fields as a SgField.RETURN_TYPE_TEXT.  Fixes issue #11.
Added support for SgFieldUrl local url types

SgFieldUrl did not support url types that were local.  Updated doc string with
new info as well.  This fixes issue #12.